1. Field of the Invention
This invention relates to soft, high water content, biomedical devices including contact lenses prepared from N-vinyl lactam monomers with or without comonomers and crosslinked with a resonance free di(alkene tertiary amine) cyclic compound.
2. Prior Art Statement
Many attempts have been undertaken to resolve the problem of polymerization and copolymerization of N-vinyl lactams with the methacrylate type comonomers. An important characteristic of the resulting polymers is their ability to absorb water and it has been a goal to obtain polymers with high water absorption characteristics while not sacrificing other properties. To date, these polymers with a water content of about 60% or more have been found to be mechanically weak and exhibit high percentages of extractibles after polymerization.
U.S. Pat. No. 3,532,679 (Steckler) describes the polymerization of N-vinyl lactams with alkyl methacrylates such as methyl, ethyl, butyl, 2-hydroxy ethyl, in the presence of a crosslinking agent which is preferably tetraethyleneglycol dimethacrylate. The resulting polymers have water absorption contents of 52% to 95%, however, they can be extracted very easily and lose their original high water content. This reversal of properties is caused by the failure of the dimethacrylates to uniformly copolymerize with the N-vinyl lactam. The hydrogel polymers of this patent are disclosed as suitable for dentistry, surgery, ophthalmology and similar applications.
U.S. Pat. No. 4,022,754 (Howes et al) discloses a copolymer of 3-methoxy-2-hydroxypropyl methacrylate (G-MEMA) and N-vinyl lactams crosslinked by a class of di- or multi-functional monomers such as allyl methacrylate or 3-allyloxy-2-hydroxypropyl methacrylate to improve the mechanical strength of the polymer. The copolymers are disclosed as being useful as contact lenses. However, the water content of these copolymers remains low, around 55%. Subsequently, U.S. Pat. No. 4,036,814 (Howes et al) discloses that when the N-vinyl lactam monomer is copolymerized with an aryl or an aryloxy acrylate, or methacrylate or the corresponding amide, a water content of 80% with retention of reasonable mechanical properties can be achieved. The disclosed copolymers include benzyl methacrylate and phenoxyethyl methacrylate. The previously disclosed crosslinking agents are also employed with the new comonomer. Unfortunately, no oxygen permeability data for these polymers is disclosed.
U.S. Pat. No. 3,772,235 (Stamberger) discloses using glycidyl methacrylate, glycidyl acrylate or glycidyl crotonate as the crosslinking agent for a hetrocyclic N-vinyl monomer such as N-vinyl pyrrolidinone to obtain transparent hydrogels suitable for optical lenses. The polymers thus prepared have water contents ranging from 30% to 70%. Stamberger further discloses in U.S. Pat. No. 3,787,380 the addition of a second comonomer such as methyl methacrylate to obtain a water absorption of 60% to 83%. The polymers are stated to be machinable but no mechanical or oxygen permeability data is presented.
U.S. Pat. No. 3,759,880 (Hoffmann et al) discloses the preparation of poly-N-vinyl pyrrolidinone-2 by polymerizing vinyl pyrrolidinone in the presence of 0.5 to 10 weight percent of a cyclic acid amide containing at least two ethylenically unsaturated groups and an oxidizable metal. Suitable acid amides include N,N'-divinyl ethylene urea and N,N'-divinyl propylene urea. The resulting insoluble and only slightly swellable polymers are useful for clarifying beer, wine and fruit juices.
U.S. Pat. No. 3,992,562 (Denzinger et al) and U.S. Pat. No. 4,013,825 (Denzinger et al) disclose variations of the above Hoffmann et al process in which selected sulfur compounds or ketocarboxycyclic acids or esters are respectively substituted for the oxidizable metal. Another patent to the same assignee, U.K. Pat. No. 1,511,716, discloses the use of similar polymers and copolymers in the field of coatings where divinyl ethylene urea provided better abrasion resistance. None of the examples in these four patents shows the possibility of fabricating an optically clear polymer suitable for contact less manufacturing nor suggests properties associated with polymers used in such an application.
U.S. Pat. No. 3,949,021 (Kunitomo et al) discloses improving the weak mechanical properties of N-vinyl pyrrolidinone or a combination of N-vinyl pyrrolidinone and other vinyl monomers by simultaneously polymerizing and crosslinking in the presence of soluble linear-polymers such as poly(methyl methacrylate) and monomers such as diallyl phthalate, ethylene glycol diacrylate, hexamethylene bismaleimide, divinyl benzene and divinyl urea. The crosslinking agents having allyl groups are preferred. The resulting polymers have a water absorption of 60% to 90% and are useful for contact lens purposes. Divinyl urea is mentioned among the crosslinking agents, however, it was found that this compound does not polymerize efficiently as suggested by this patent.
Polymerization of N,N'-divinyl urea was studied by C. G. Overberger et al, J. Pol. Sci., Pt A-1, vol. 7, 35-46 (1969). It was found that although the divinyl urea exists at room temperature in its vinyl form, the tautomeric form (1)--CO--N.dbd.CH--CH.sub.3, is formed upon heating. The resulting polymer has the structure ##STR1## Thermal polymerization of divinyl urea results in soluble (non-crosslinked) polymers. Photo-initiated polymerization of divinyl urea gave similar products. Finally, radical-initiated polymerization of divinyl urea resulted in insoluble material in lower yield. However, the infrared spectrum and elemental analysis showed all three products to be identical.
U.S. Pat. No. 4,184,992 (Hosaka) summarizes the disadvantages of the prior art N-vinyl lactam base polymers for contact lenses. It was found that those previous art polymers became opaque, distorted upon immersing in boiling water and contained water-soluble extractibles. Hosaka's object was to provide a crosslinking agent reactive with the rest of the monomers to produce a hydrogel having a minimum extractibles and no change in boiling water. The object is more nearly achieved by the use of crosslinking agents with a vinyl or allyl functionality (similar in concept to U.S. Pat. No. 4,036,814) such as vinyl methacrylate, divinyl succinate, triallyl isocyanurate. In the case of polymers having water contents of 68% to 70% the amount of extractibles after 16 hours immersion in boiling water was diminished to between 5% and 10%. In the case of polymers having a water content of 73%, the comparable result was 7% to 9%.
U.S. Pat. No. 4,158,089 (Loshaek et al) introduces allylic monomers such as mono and diallyl itaconate, diallyl maleate, diallyl fummarate as crosslinking agents for N-vinyl pyrrolidinone/alkyl acrylate or methacrylate polymers and compares the properties of these polymers with poly(hydroxyethyl methacrylate) commercially available under the tradename Dura-Soft. Depending on the amount of crosslinking agent, diallyl itaconate, the polymers exhibit water content between 60% and 98% with extractibles of 9% to 20%. The polymer's mechanical strength rating is much inferior to poly(hydroxyethyl methacrylate) which is rated 10, a polymer with a strength rating of 5 has a 57% water content, a polymer with a strength rating of 3 has a 71% water content and a polymer having a strength rating of 3 to 4 has an 80% water content. To achieve low extractibles, 10% or lower at high water content, usually high amounts of crosslinking agent is necessary. The high amount of crosslinking agent causes the formation of rigid polymers having low strength rating. U.S. Pat. No. 4,182,802 (Loshaek et al) discloses a further attempt to improve the property of this type of polymer by the incorporation of styrene as a comonomer. Again, as long as the amount of crosslinking agent is low, higher extractibles, 10% to 18% resulted.
From the foregoing, it can be appreciated that there is still a need for an optically clear polymer of N-vinyl lactam which has low extractibles, good mechanical properties (especially the tear strength), high oxygen permeability and good machinability for use in various biomedical applications.